Photographic processes for producing direct-positive images without requiring a reversal processing step or a negative film are well known.
With the exception of those using special materials, methods commonly used for producing positive images employing known direct-positive silver halide photographic materials can be divided generally into two types on the basis of their practical usefulness. The first method uses prefogged silver halide emulsions, and produces direct-positive images after development through destruction of fogged nuclei (latent images) utilizing solarization, Herschel effect or the like. The second method uses nonfogged silver halide emulsions of the internal latent-image type, and produces direct-positive images by surface development after or during fogging processing subsequent to imagewise exposure. The term "silver halide emulsions of the internal latent-image type" as used above refers to silver halide photographic emulsions containing light-sensitive nuclei mainly inside the silver halide grains, that form a latent image mainly inside the grains upon exposure. Such methods of the second type generally provide high sensitivity, compared with those of the first type, and are suitable for uses in which high sensitivity is required. The present invention relates to a method of this type.
Various methods of the second type are known in the art, including the principal processes described, e.g., in U.S. Pat. Nos. 2,592,250, 2,466,957, 2,497,875, 2,588,982, 3,317,322, 3,761,266, 3,761,276 and 3,796,577, British Pat. Nos. 1,151,363 and 1,150,553. By these known methods, direct-positive photographic light-sensitive materials having relatively high sensitivity can be produced.
Details of the mechanism of direct-positive image formation are described, e.g., in T. H. James, The Theory of the Photographic Process (4th ed., 1977), Chap. 7, pp. 182-193, and U.S. Pat. No. 3,761,276. Specifically, it is believed that the surface desensitization attributable to internal latent images produced inside silver halide grains by the first imagewise exposure results in selective formation of fogged nuclei only on the individual surfaces of silver halide grains present in unexposed areas, and that conventional surface-development processing produces a photographic direct-positive image in the unexposed areas.
In order to selectively form fogged nuclei, as described above, a second exposure can be applied to the whole surface of a light-sensitive layer, by what is generally called an "optical fogging method" (described, e.g., in British Pat. No. 1,151,363), or a nucleating agent can be used in a "chemical fogging method" (described, e.g., in Research Disclosure, Vol. 151, No. 15162, pp. 76-78 (November 1976)).
Formation of direct-positive color images can be achieved by subjecting silver halide photographic materials of the internal latent-image type to surface color development-processing simultaneous with or subsequent to a fogging treatment, followed by bleach processing and then fixation processing, or by bleach-fix processing. After bleaching and fixing the material is washed and/or stabilized.
In a chemical fogging method which has conventionally been employed in the above-described process, compounds capable of acting as nucleating agents only in a high pH range above 12 have been used. Under such high pH conditions, developing agents are subject to deterioration due to air oxidation, and consequently, the chemical fogging method has the defect of being accompanied by a significant decrease in development activity. In addition, a decrease in the rate of development requires increased processing time. Using a developer at a lower pH is particularly undesirable, because it necessitates a still longer processing time. Even in the pH range above 12, development requires a long time for completion.
In contrast, optical fogging methods are relatively advantageous in practical use, since they do not require high pH conditions. However, there are various technical problems in applying such methods to widely varying photographic purposes. Specifically, the optical fogging method is based on the formation of fogged nuclei through photolysis of silver halides, so that correct illuminance and correct exposure depend on the kind of silver halide used and its characteristics. Therefore, it is difficult to achieve consistent results in fogging silver halides. In addition, the method has the disadvantages of requiring a complex and expensive developing apparatus, as well as long development time.
As described above, it is difficult for either conventional fogging method to provide direct-positive images of excellent quality rapidlly and consistently. In order to overcome these problems, the use of compounds exhibiting nucleating capabilities even at a pH below 12 is proposed in Japanese Patent Application (OPI) No. 69613/77 (the term "OPI" as used herein means an "unexamined published Japanese patent application"), and U.S. Pat. Nos. 3,615,615 and 3,850,638. However, those nucleating agents have an adverse effect such that they act on silver halides or are subject to decomposition during storage of photosensitive materials before photographic processing, resulting in lower maximum density of the developed images.
It is disclosed in U.S. Pat. No. 3,227,552 that an increase in development rate can be achieved in the medium density region by using specified hydroquinone derivatives. However, such development rates remain insufficient, particularly when developers adjusted to a pH below 12 are used.
It is also disclosed in Japanese Patent Application (OPI) No. 170843/85 that maximum image density can be increased by the addition of mercapto compounds containing a carboxylic acid group or a sulfonic acid group. However, the effect on increase of maximum image density produced by these compounds is also inadequate.
Further, Japanese Patent Application (OPI) No. 134848/80 indicates that minimum image density can be decreased by carrying out development processing in the presence of a nucleating agent using a processing solution (pH 12.0) containing a tetraazaindene-series compound that prevents rereversal negative-image formation. However, neither maximum image density nor development rate is increased by this method.
In addition, Japanese Patent Publication No. 12709/70 proposes the addition of a triazoline thione-series compound or a tetrazoline thione-series compound as an antifoggant to a photosensitive material capable of forming a direct-positive image using an optical fogging method. However, this method also fails to provide high maximum density or a high development rate.
No method has yet been discovered capable or rapidly forming direct-positive images while furnishing both satisfactory high maximum density and satisfactory low minimum density. Such a method is particularyl desired in instant color photography (a color material diffusion transfer process) which requires image formation in a short time. Here as well, a quicker method of image formation is eagerly sought.
Furthermore, it is generally known such a disadvantage that the higher the sensitivity of direct-positive emulsion is, the more the generation of rereversal of negative image under high intensity exposure is.